some latex symbol updates

[JeffBezanson]

• Added \emptyset alias for ∅
• The double bracket characters seemed to be using fullwidth versions. I switched them to the normal math codepoints. I also call these \llbracket and \rrbracket in latex, but maybe I'm using a different package than most people.

[JeffBezanson]

There's a similar issue with overbrace and underbrace. They should probably use "U+23DE TOP CURLY BRACKET" and "U+23DF BOTTOM CURLY BRACKET". This would be more consistent with \overbracket which uses "U+23B4 TOP SQUARE BRACKET".

[carlobaldassi]

The table in the docs should be updated too, once the changes are settled.

[JeffBezanson]

Also, my suggestions are consistent with unicode-math-table.tex. I think unicode.xml is just wrong here.